Charles Ellicott Commentary Exodus 20:25

"And if thou make me an altar of stone, thou shalt not build it of hewn stones; for if thou lift up thy tool upon it, thou hast polluted it." — Exodus 20:25 (ASV)

If thou wilt make me an altar of stone. —Among civilized nations altars were almost always of stone, which superseded earth, as more durable. God does not absolutely prohibit the employment of stone altars by the Israelites, who are found to use them upon certain occasions (Joshua 8:31; 1 Kings 18:32). He is content to forbid the shaping of the stones by an implement, so that they may not give rise to idolatry. (See Note on Exodus 20:24.)

Thou hast polluted it. —Nature is God’s handiwork, and, therefore, pure and holy. Man, by contact with it, imparts his impurity to it. The altar, by which sin was to be expiated, required to be free from all taint of human corruption. For the construction of the altar afterwards sanctioned, see the comment on Exodus 27:1.